How do you correct the speedometer for larger tires? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72c10lover16 · 11-03-2011, 03:57 AM

I have a 68 chevy pickup with a 4 speed in it. it is a 4 wheel drive pickup. i have 33 inch tires on it and i was wondering how do you correct the speedometer for the larger tires?

webfoot · 11-03-2011, 09:24 PM

The speedo gear in the transfer case will have to be changed out. Not sure of a source though. If it is too far off you will also need to get a different drive gear. There are 2 parts; drive gear on the output shaft and the driven gear which is where the cable attaches to.

72c10lover16 · 11-03-2011, 09:44 PM

Does anybody know what spedo gear I would need. My speed is off by about ten doing interstate speed
Posted via Mobile Device

webfoot · 11-03-2011, 10:01 PM

10% or 10 mph? Need to drive by a radar point and note the difference, otherwise you'll just be guessing.

72c10lover16 · 11-03-2011, 10:09 PM

I'm not guessing. DOing 65 on the spedo is really 75
Posted via Mobile Device

Todersauce · 11-04-2011, 05:38 AM

I use an old GPS most have a speed display its not fast to respond in traffic but they work well on the highway and its cheaper then a big bill from the trans shop.

OrrieG · 11-04-2011, 03:00 PM

This might help:

http://www.chevytalk.org/fusionbb/sh...hp?tid/256975/

rayfinseats · 11-30-2011, 02:12 AM

Try a local chevy dealer parts counter. They usually can tell you.

**special-K** · 11-30-2011, 09:07 AM

33s don't even throw my speedo off enough to matter. My experience with 37s has been it's off about 10%. I've had several vehicles with both. You must have the wrong gear for your ratio even with stock tires or something's wrong with your speedo. The worst I've seen was running 37s on 3/4t trucks that would have 31s originally. Which was 10%,like 70 read around 63 or 55 reads 50.
On my '85 Jimmy that came with 3.07s and short little 235/75-15s that I now have 4.10s and 35s with nothing changed with the speedo it reads about right up to about 50,then shoots up to 75-80. I haven't figured that one out yet,but figure it's something in my speedo.

coondogg52 · 12-01-2011, 02:06 PM

I am running on 33" tires and my speedo is off 5 mph when at freeway speed, but as I go slower the speedo tends to get closer to true.

skoffice · 12-02-2011, 05:09 AM

I have the exact same problem with my 33's. Here locally there is Sacramento Speedo that I need to look into visiting very soon. I think there is a larger source for parts in Nevada as well. I believe Nevada Speedometer is the name of company. Very original business names

**special-K** · 12-03-2011, 07:26 AM

^ What city is Sacramento Speedo located in?

thirdstreettito · 12-12-2011, 11:13 AM

My 88 has 31's on it. Factory was 235/75R15(28"). When it says 55mph I am really doing 60mph. I got it checked at a local shop that has the rollers.

Gene · 12-12-2011, 11:47 PM

I'm running 35's and mine is off 9mph.
Posted via Mobile Device

11-03-2011, 03:57 AM	#1
72c10lover16 Registered User Join Date: May 2008 Location: Laramie, Wyoming Posts: 93	How do you correct the speedometer for larger tires? I have a 68 chevy pickup with a 4 speed in it. it is a 4 wheel drive pickup. i have 33 inch tires on it and i was wondering how do you correct the speedometer for the larger tires? __________________ 68 swb 4x4 327 4 speed

11-03-2011, 09:24 PM	#2
webfoot Registered User Join Date: Nov 2007 Location: Hermiston, OR Posts: 947	Re: How do you correct the speedometer for larger tires? The speedo gear in the transfer case will have to be changed out. Not sure of a source though. If it is too far off you will also need to get a different drive gear. There are 2 parts; drive gear on the output shaft and the driven gear which is where the cable attaches to. __________________ *Running in place gets you nowhere fast.* -me 1968 GTO 4-spd covnvertible 1970 K-20 4-speed, factory AC.

11-03-2011, 09:44 PM	#3
72c10lover16 Registered User Join Date: May 2008 Location: Laramie, Wyoming Posts: 93	Re: How do you correct the speedometer for larger tires? Does anybody know what spedo gear I would need. My speed is off by about ten doing interstate speed Posted via Mobile Device __________________ 68 swb 4x4 327 4 speed

11-03-2011, 10:01 PM	#4
webfoot Registered User Join Date: Nov 2007 Location: Hermiston, OR Posts: 947	Re: How do you correct the speedometer for larger tires? 10% or 10 mph? Need to drive by a radar point and note the difference, otherwise you'll just be guessing. __________________ *Running in place gets you nowhere fast.* -me 1968 GTO 4-spd covnvertible 1970 K-20 4-speed, factory AC.

11-03-2011, 10:09 PM	#5
72c10lover16 Registered User Join Date: May 2008 Location: Laramie, Wyoming Posts: 93	Re: How do you correct the speedometer for larger tires? I'm not guessing. DOing 65 on the spedo is really 75 Posted via Mobile Device __________________ 68 swb 4x4 327 4 speed

11-04-2011, 05:38 AM	#6
Todersauce Registered User Join Date: Nov 2009 Location: Idaho Posts: 34	Re: How do you correct the speedometer for larger tires? I use an old GPS most have a speed display its not fast to respond in traffic but they work well on the highway and its cheaper then a big bill from the trans shop.

11-04-2011, 03:00 PM	#7
OrrieG Registered User Join Date: Jun 2006 Location: Idaho Posts: 8,800	Re: How do you correct the speedometer for larger tires? This might help: http://www.chevytalk.org/fusionbb/sh...hp?tid/256975/ __________________ 1959 Chevy Short Fleetside w/ 74 4WD drive train (current project) OrrieG Build Thread 1964 Chevelle Malibu w/ 355-350TH (daily driver) Helpful AD and TF Manual Site Old Car Manual Project

11-30-2011, 02:12 AM	#8
rayfinseats Registered User Join Date: Dec 2010 Location: sioux city, iowa Posts: 619	Re: How do you correct the speedometer for larger tires? Try a local chevy dealer parts counter. They usually can tell you.

11-30-2011, 09:07 AM	#9
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: How do you correct the speedometer for larger tires? 33s don't even throw my speedo off enough to matter. My experience with 37s has been it's off about 10%. I've had several vehicles with both. You must have the wrong gear for your ratio even with stock tires or something's wrong with your speedo. The worst I've seen was running 37s on 3/4t trucks that would have 31s originally. Which was 10%,like 70 read around 63 or 55 reads 50. On my '85 Jimmy that came with 3.07s and short little 235/75-15s that I now have 4.10s and 35s with nothing changed with the speedo it reads about right up to about 50,then shoots up to 75-80. I haven't figured that one out yet,but figure it's something in my speedo. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ Last edited by special-K; 12-13-2011 at 08:45 AM.

12-01-2011, 02:06 PM	#10
coondogg52 Registered User Join Date: Dec 2010 Location: modesto,ca Posts: 10	Re: How do you correct the speedometer for larger tires? I am running on 33" tires and my speedo is off 5 mph when at freeway speed, but as I go slower the speedo tends to get closer to true.

12-02-2011, 05:09 AM	#11
skoffice Registered User Join Date: Apr 2007 Location: Elk Grove, CA Posts: 375	Re: How do you correct the speedometer for larger tires? I have the exact same problem with my 33's. Here locally there is Sacramento Speedo that I need to look into visiting very soon. I think there is a larger source for parts in Nevada as well. I believe Nevada Speedometer is the name of company. Very original business names __________________ 1964 C10 Shortened Shortbed Cammed LQ9 P&P 1967 C10 Trophy Truck BB 438 (soon to be worked on again)

12-03-2011, 07:26 AM	#12
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: How do you correct the speedometer for larger tires? ^ What city is Sacramento Speedo located in? __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

12-12-2011, 11:13 AM	#13
thirdstreettito Resident of Here Join Date: May 2008 Location: Wesley Chapel, FL, USA Posts: 7,716	Re: How do you correct the speedometer for larger tires? My 88 has 31's on it. Factory was 235/75R15(28"). When it says 55mph I am really doing 60mph. I got it checked at a local shop that has the rollers. __________________ ~Bradley~ 2004 Black GTO LS2/T56 too much to list Miss having a truck

12-12-2011, 11:47 PM	#14
Gene Registered User Join Date: Aug 2002 Location: Muskogee, Oklahoma Posts: 2,914	Re: How do you correct the speedometer for larger tires? I'm running 35's and mine is off 9mph. Posted via Mobile Device __________________ 1971 SWB 4X4 Build BOARD MEMBER #5448 BOOMER SOONER!!!!



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